Abstract
Abstract— Solid solution hardening and strain hardening are the dominating strengthening mechanisms to achieve a high yield stress and ultimate tensile strength in Cr‐Mn‐N steels for drill collars. The fatigue limit can be improved more effectively by solid solution hardening than by strain hardening, but the attainable hardness due to soluble elements is restricted by other metallurgical demands. Strain hardening significantly increases the strength and the fatigue limit is improved too, but to a lesser extent. The reason for this behaviour is the introduction of internal stresses of the I, II and III kind by forging the drill collars. This results in macroscopic stresses which are varying over the cross section and in stresses of a microscopical scale which lead to an early loss of linearity in the elastic line of the stress strain curve and to the well known Bauschinger effect. Cyclic softening and hardening is accompanied by a rearrangement of the dislocation structure as revealed by transmission electron microscopy. The results indicate that a multiaxial and homogeneous cold working to produce a stable dislocation structure and to avoid large directional internal stresses is extremely important to achieve high fatigue strengths. Crack growth and crack closure measurements were performed for determination of the effective cyclic threshold stress intensity range for evaluation of the influence of the grain size on the fatigue limit.
Published Version
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